The invention relates to calibration of vector or quadrature (I/Q) modulators, in particular to calibration of a vector modulator using only one scalar detector.
A major trend in microwave communications is the switch to digital communication, using digital modulation techniques. These techniques require a new class of signal modulators which perform vector or quadrature modulation. A vector modulated signal is an amplitude and phase modulated signal that is modulated according to in-phase and quadrature phase components, which define a phase vector. In vector modulation, two modulation input signals independently modulate the in-phase (I) and quadrature (Q) components of the carrier. For proper operation, the I and Q channels of the modulator must be calibrated to be equal in gain ,i.e., balanced, and precisely 90.degree. apart, i.e., in quadrature.
One way to calibrate a vector modulator is with a network analyzer connected to the RF carrier input and to the RF modulated output. The network analyzer measures the amplitude and phase of the RF modulated output resulting from varying DC voltages applied to the I and Q modulation inputs. From these measurements, the gain and phase accuracy of the modulator can be determined, and calibrated. The drawback with this method is that it is an expensive and somewhat cumbersome solution, and therefore it does not lend itself to incorporation into a vector modulator for self calibration. It is also limited in accuracy for most network analyzers.
An object of the present invention is to provide apparatus and a method for calibrating a vector modulator that does not require a network analyzer, but instead uses a scalar RF detector. This improves the accuracy of the calibration measurement and makes the calibration system compact and inexpensive enough to be incorporated in the vector modulator for self calibration.
The present invention accomplishes this and other objectives with a system comprising a standard vector modulator circuit modified by adding phase shifters to adjust the relative phases of the I and Q components of the RF carrier. Variable attenuators are inserted in the I and Q modulation signal input lines to adjust the relative amplitude of the modulation signals.
Calibration measurements are made using a scalar detector to measure the amplitude of the RF output signal. Two DC signal sources provide reference signals for the I and Q modulation inputs. Two other DC signal sources provide carrier leak compensation signals. A final DC signal source 239 provides calibration signals for balancing the amplitude of the I and Q modulation signals.
An iterative four step calibration process is followed until no change in the results is observed. The quadrature phase error is minimized by adjusting the phase shifters. The carrier leakage is minimized by adjusting the carrier leak compensation sources to minimize RF output with the modulation inputs grounded. The amplitudes of the I and Q modulation signals are balanced by adjusting the attenuators until the output amplitudes are equal. Finally, the quadrature calibration signal sources are adjusted until the output amplitudes they produce are balanced. This series of adjustments is repeated until no further changes are observed.